Machining of titanium based alloys is generally characterized by that the tool life is short and the rate of metal removal is low. This is due to high temperatures and unfavourable temperature distribution in the tools. Wet machining is normally used in order to minimise the generation of heat, thereby increasing the tool life.
Titanium based alloys are generally divided into three groups of alloys: alpha, alpha+beta and beta alloys. Alloying elements such as Al, V, Sn stabilise the various types of alloys and modify their properties.
Uncoated cemented carbide cutting inserts have traditionally been used for the machining of titanium based alloys. However, during recent years it has been more common to use coated carbide substrates, where the substrate has a relative high cobalt-content. For example, EP1307602A1 discloses a chromium containing cemented carbide substrate consisting of between 10.4 and 12.7 weight percent (wt %) cobalt, between 0.2 and 1.2 wt % chromium, and further tungsten and carbon. The substrate has a coating including a base layer of titanium carbonitride.
WO2009/035396A1, the content of which is hereby incorporated by reference, discloses a cutting tool insert, particularly useful for machining of titanium based alloys. The insert comprises a cemented carbide substrate and a coating. The cemented carbide has a composition of 4-14 wt % Co, up to 5 wt % of the elements Ti, Ta and/or Nb and/or Cr or other elements from groups IVB, VB or VB and rest WC. The coating is composed of one or more layers of refractory compounds of which at least one layer comprise crystalline cubic NbN.
EP1722009A1 discloses a cutting tool insert, solid end mill, or drill, comprising a substrate and a coating composed of one or more layers of refractory compounds of which at least one layer comprises a h-Me1Me2X phase, where Me1 is one or several of the elements V, Cr, Nb, and Ta, and Me2 is one or several of the elements Ti, Zr, Hf, Al, and Si and X is one or several of the elements N, C, O or B.
U.S. Pat. No. 5,325,747 discloses a coated cutting tool insert comprising a substrate and a coating comprising at least one CVD layer, which may have a residual tensile stress, and at least one layer having a high residual compressive stress. The outermost layer of the coating is preferably the one containing residual compressive stresses. The substrate is a composite having hard refractory grains. The cutting tool has been found to be particularly useful in the machining of titanium alloys and super alloys.
Although earlier coated cutting inserts have satisfactory performance, it is still a desire to provide a cutting insert that has improved cutting properties when it comes to withstanding mechanical shocks and thermal shocks during machining of a work piece, especially in a milling operation.